Thus, an actuator, commonly known as a pneumatic actuator, comprises an actuator piston disc that is displaceable in axial direction between a first position (inactive position) and a second position (active/extended position). The displacement is achieved by controlling a supply of pressure fluid, such as pressurized gas/air, that acts on the actuator piston disc. The actuator piston disc in turn directly or indirectly acts on the object that is to be displaced, for example an engine valve, for controlling its position.
When the actuator piston disc is in the inactive position the engine valve is in contact with its seat, and when the actuator piston disc is in the active position the engine valve is open, i.e. situated at a distance from its seat.
In the applicants own document WO 2013-058704 an actuator is described in which a pressure pulse, which is started by a first inlet valve body opening and allowing pressure fluid from a pressure fluid source to act on and drive the actuator piston disc from its resting position, is stopped by a second inlet valve body, that is rigidly connected to and jointly displaceable with the actuator piston disc, cutting the flow from the pressure fluid source and thereby closes the inlet channel. This construction provides a direct correlation between the pulse length and the travel that the actuator piston disc performs.
Nevertheless, the valve bodies that open/close the inlet channel and the outlet channel have in this publication relatively large mass and small throughput areas. It is also known that some applications demand high working pressure/high pressure, for example 20-25 Bar, to achieve a correct function of the actuator, i.e. to unction together with internal combustion engines with a range of number of turns up to 8-10 thousand turns per minute. There is further a wish to avoid that the temperature rises in the actuator and the surrounding parts/fluids in such applications as a result of the very operation of the actuator and the accompanying compressor, and this is achieved by holding a pressure relation low and thereby a so called enhanced return pressure is used, also known as low pressure/base pressure. In other words the pressure of the pressure fluid that is located downstream from the actuator and upstream from the compressor is much higher than the atmospheric pressure, for example 4-6 Bar. The relatively large mass of the valve bodies results in that the valve bodies risk to rebound from their seats when they shall place themselves in their respective resting positions, whereby jarring and vibrations arise and/or the included parts are damaged, and lead to imprecise control of the pressure fluid in the inlet channel and the outlet channel, respectively.
The relatively small throughput areas in combination with the high return pressure lead to, at the return of the actuator to the inactive position, the evacuation of the pressure fluid from the cylinders first portion risks to be insufficient, which leads to a slow return of the actuator piston disc.